1. Field of the Invention
The subject matter of the present invention is a method for the production of benzaldehydes, in which the corresponding benzyl halides are oxidized to the aldehyde.
2. Discussion of Prior Art
It is known to perform the oxidation of benzyl halides to benzaldehydes by means of organic oxidants. The disadvantage of this method lies in the long reaction times and the low yields. Furthermore, the working up of the reaction products often involves great difficulty.
Trimethylamine-N-oxide has already been proposed as an organic oxidant for the transformation of unsubstituted benzyl bromide to benzaldehyde [Ber. 94 (1961), p. 1360]. A disadvantage of this procedure is the use of a great excess of aminoxide, which has to be used in the anhydrous state, inasmuch as it has a tendency toward spontaneous, explosive decomposition. Furthermore, this method also has the disadvantage of the low yield of under 50%.
It is furthermore known to use pyridine-N-oxide as an organic oxidant in the oxidation of benzyl halides to benzaldehydes [J. Org. Chem. 22 (1957), p. 1135]. A disadvantage in this procedure is the fact that the quaternary salt forming as an intermediate from pyridine oxide and the benzyl halide is stable and it can be cleaved to benzaldehyde and pyridine only by an additional treatment with dilute caustic soda solution. In this procedure, which also has to be performed with anhydrous pyridine oxide, the yields are only around 40%.
The problem, therefore, existed of performing the oxidation of benzyl halides to benzaldehydes technically such that high yields of pure product are obtained, the reaction being performed in a single step, if possible, and with easily available compounds. Furthermore, the desired process should be applicable especially to the production of benzaldehydes substituted in the nucleus, whose preparation is made more difficult in the case of an ortho substitution of electron-attracting substitutuents.